1. Field of the Invention
The present invention relates generally to microwave applicators for heating an electrically-conductive workpiece and more particularly to microwave applicators for heating an electrically-conductive workpiece wherein the applicator and the workpiece need not be in contact and the workpiece may be external to the microwave applicator.
2. Description of the Prior Art
In many modern applications, composite materials are utilized to produce structures which are lightweight, but have the same strength as similar structures composed of metals or other alternative materials. A composite material usually consists of high-strength fibers embedded in a resin matrix that binds the fibers together into the form required for the structural function. Frequently, the most useful fibers are made of carbon, a good electrical conductor.
The use of such composite materials has been generally restricted to relatively small structures due to the requirements of the composite materials manufacturing process which typically utilizes an autoclave or press in which the composite structure must fit in order to bind the individual sheets or ribbons of composite material together to form the finished part. Thus, the size of composite parts has generally been limited by the size of the autoclaves and presses available.
Microwave heating has been employed as an alternative heat source in forming composite parts since it can be instantaneously turned on or off as well as varied in amount as required by the process. However, most attempts have utilized a microwave oven type device, typically referred to as a multimode cavity, which suffered from several deficiencies. Once again, the size of the microwave oven cavity limited the size of the composite parts capable of being processed since the parts were required to be placed inside the oven. Additionally, composite parts which utilized conductive fibers reflected the microwaves so that the conductive material was heated little if at all. Composite parts formed from conductive materials also caused significant arcing and sparking when heated with microwave radiation.
An alternative method of microwave heating is accomplished by having an open-ended microwave applicator which is externally passed over the surface of the object to be heated so as to heat the workpiece with the microwave energy radiating from a cavity of the applicator. A typical microwave applicator which has an open cavity is shown in U.S. Pat. No. 4,392,039 (hereinafter the '039 patent), Dielectric Heating Applicator, issued to Per O. Risman on Jul. 5, 1983. The '039 patent is generally only applicable to the heating of surfaces which are not electrically conductive. This limited application is due to the heating in the '039 patent being accomplished by dielectric losses in the irradiated material with the amount of heating dependent on the penetration depth of the energy into the material. Thus, the use of dielectric heating for conductive materials, such as carbon fiber composite materials, would be ineffective due to the large electrical conductivity of the conductive material which would only allow insubstantial penetration by the microwave energy and would reflect the energy back toward the microwave source so that the conductive material would be heated very little.
Furthermore, a common problem of open cavity microwave applicators in heating electrically conductive workpieces is the necessity to maintain electrical contact between the applicator and the object to be heated to allow electrical currents to flow between the cavity walls of the applicator and the object. If contact is not maintained, damaging arcing over the gap could occur as well as substantial lowering of the energy transfer efficiency and variance of the energy flow so as to cause uneven heating. Additionally, controlling the position of the microwave applicator so as to maintain electrical contact with the object to be heated without applying excessive pressure to the workpiece so that the applicator does not scrub the surface of the workpiece and impart undesirable finish marks is difficult and becomes increasingly more so as the speed with which the applicator moves over the object's surface increases.
It would be desirable to develop an open-ended cavity microwave applicator for heating electrically conductive materials which did not require electrical currents to pass from the applicator to the material being formed so that the applicator does not need to be in constant contact with the object to be heated in order to avoid undesirable finish marks, while not suffering from arcing, decreased energy transfer efficiency, and varied energy flow. Furthermore, it would be desirable to develop and open-ended cavity microwave applicator which did not limit the dimensions of the workpiece on which the applicator was capable of heating.